As is known in the art, it is desirable in many applications to accurately determine the spatial position (and sometimes the angular orientation, as well) of the distal tip of a flexible instrument. For example, in brachytherapy, it is necessary to implant a number of precisely spaced radioactive seeds into cancerous tumors inside the human body with a high degree of accuracy so that the radiation can be concentrated on the diseased tissue with a minimum of damage to surrounding healthy tissue.
The use of implantation needles to insert radioactive seeds is well known in the art, but these methods require a high degree of rigidity in the implantation needle to deliver the seeds accurately, as the target locations are assumed to lie on the needle's longitudinal axis. Since these needles are also necessarily thin to minimize tissue damage when driven into the body, they often are insufficiently rigid and so require elaborate support mechanisms to guide their insertion and to keep them as straight as possible. Despite this effort, bending of the needle occurs as it travels through the tissue, thus introducing substantial positional errors.
These errors could be reduced considerably if the end (or distal) tip of the needle could be accurately measured relative to the target location, such that any bending in the needle could be monitored, and compensation applied.
Other applications of such an instrument include, for example, catheters, endoscopes, and bronchoscopes, which are sufficiently thin and flexible to navigate interior passages of the human body.
General purpose instruments have been developed that incorporated bend and twist sensors distributed along their length at known intervals. These bend and twist sensors allow the user to approximate the tip position of the device by monitoring the manner in which the device “bends” and “twists” as it is moved in 3D space. A sensor data processing system is coupled to these bend and twist sensors and receives the flexure signals from these sensors. The processing system monitors the bend and twist sensors disposed along the device and extrapolates the device and tip position. This type of a system is known as a path-dependent measuring system; i.e., a system that requires knowledge of the spacing between each pair of sensors and a signal from each sensor to perform an extrapolation to determine the device's orientation. Particularly important is that, for path-dependent systems such as this, the distal end tip position is determined successively from intermediate measurements along the length of the flexible structure, beginning at a known location, typically the proximal end.
Path-dependent measurements of the distal end tip of a long thin flexible structure suffer from the inherent limitations of the end tip measurement being dependent upon a long chain of extrapolations from preceding measurements along the path. Small amounts of error in each location and orientation along the path can rapidly accumulate, resulting in a large buildup of error by the time the end of the path is reached.
The distal end tip of a thin flexible structure can also be determined by affixing a path-independent sensor to the distal end, such as a magnetic sensor, whose position can be tracked by a magnetic system employing a magnetic field generator, or an optical marker, whose position can be tracked by an optical system employing a number of light sensors. Such path-independent measurement devices provide direct position measurements and so are free of the extrapolation error accumulation inherent in path-dependent measurements, but they are subject to other limitations. For example, an optical distal end marker could not be used where the distal end was outside of the optical sensors' line of sight, such as in a medical instrument designed for insertion inside a patient's body, while a magnetic sensor would be subject to errors from magnetic disturbances and would be typically limited to smaller operational volumes. Such path-independent measurement devices do not have the robustness inherent in path-dependent devices employing several sensors.